Ideal oxygen precipitating epitaxial silicon wafers and oxygen out-diffusion-less process therefor

ABSTRACT

A process for preparing an silicon epitaxial wafer. The wafer has a front surface having an epitaxial layer deposited thereon, a back surface, and a bulk region between the front and back surfaces, wherein the bulk region contains a concentration of oxygen precipitates. In the process, a wafer having interstitial oxygen atoms is first subjected to an oxygen precipitation heat treatment to cause the nucleation and growth of oxygen precipitates to a size sufficient to stabilize the oxygen precipitates. An epitaxial layer is then deposited on the surface of the oxygen precipitate stabilized wafer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/340,489, filed Jun. 30, 1999, and now U.S. Pat. No. 6,204,152 whichis a divisional of U.S patent application Ser. No. 08/806,436 and nowU.S. Pat. No. 5,994,761, filed on Feb. 26, 1997.

BACKGROUND OF THE INVENTION

The present invention generally relates to the preparation ofsemiconductor material substrates, especially silicon wafers, which areused in the manufacture of electronic components. More particularly, thepresent invention relates to a process for the treatment of siliconwafers which enables the wafers, during the heat treatment cycles ofessentially any is arbitrary electronic device manufacturing process toform an ideal, non-uniform depth distribution of oxygen precipitates.

Single crystal silicon, which is the starting material for mostprocesses for the fabrication of semiconductor electronic components, iscommonly prepared with the so-called Czochralski process wherein asingle seed crystal is immersed into molten silicon and then grown byslow extraction. As molten silicon is contained in a quartz crucible, itis contaminated with various impurities, among which is mainly oxygen.At the temperature of the silicon molten mass, oxygen comes into thecrystal lattice until it reaches a concentration determined by thesolubility of oxygen in silicon at the temperature of the molten massand by the actual segregation coefficient of oxygen in solidifiedsilicon. Such concentrations are greater than the solubility of oxygenin solid silicon at the temperatures typical for the processes for thefabrication of electronic devices. As the crystal grows from the moltenmass and cools, therefore, the solubility of oxygen in it decreasesrapidly, whereby in the resulting slices or wafers, oxygen is present insupersaturated concentrations.

Thermal treatment cycles which are typically employed in the fabricationof electronic devices can cause the precipitation of oxygen in siliconwafers which are supersaturated in oxygen. Depending upon their locationin the wafer, the precipitates can be harmful or beneficial. Oxygenprecipitates located in the active device region of the wafer can impairthe operation of the device. Oxygen precipitates located in the bulk ofthe wafer, however, are capable of trapping undesired metal impuritiesthat may come into contact with the wafer. The use of oxygenprecipitates located in the bulk of the wafer to trap metals is commonlyreferred to as internal or intrinsic gettering (“IG”).

Historically, electronic device fabrication processes included a seriesof steps which were designed to produce silicon having a zone or regionnear the surface of the wafer which is free of oxygen precipitates(commonly referred to as a “denuded zone” or a “precipitate free zone”)with the balance of the wafer, i.e., the wafer bulk, containing asufficient number of oxygen precipitates for IG purposes. Denuded zonescan be formed, for example, in a high-low-high thermal sequence such as(a) oxygen out-diffusion heat treatment at a high temperature (>1100°C.) in an inert ambient for a period of at least about 4 hours, (b)oxygen precipitate nuclei formation at a low temperature (600-750° C.),and (c) growth of oxygen (SiO₂) precipitates at a high temperature(1000-1150° C.). See, e.g., F. Shimura, Semiconductor Silicon CrystalTechnology, Academic Press, Inc., San Diego Calif. (1989) at pages361-367 and the references cited therein.

More recently, however, advanced electronic device manufacturingprocesses such as DRAM manufacturing processes have begun to minimizethe use of high temperature process steps. Although some of theseprocesses retain enough of the high temperature process steps to producea denuded zone and sufficient density of bulk precipitates, thetolerances on the material are too tight to render it a commerciallyviable product. Other current highly advanced electronic devicemanufacturing processes contain no out-diffusion steps at all. Becauseof the problems associated with oxygen precipitates in the active deviceregion, therefore, these electronic device fabricators must use siliconwafers which are incapable of forming oxygen precipitates anywhere inthe wafer under their process conditions. As a result, all IG potentialis lost.

SUMMARY OF THE INVENTION

Among the objects of the invention, therefore, is the provision of asingle crystal silicon wafer which, during the heat treatment cycles ofessentially any electronic device manufacturing process, will form anideal, non-uniform depth distribution of oxygen precipitates; theprovision of such a wafer which will optimally and reproducibly form adenuded zone of sufficient depth and a sufficient density of oxygenprecipitates in the wafer bulk; the provision of such a wafer in whichthe formation of the denuded zone and the formation of the oxygenprecipitates in the wafer bulk is not dependent upon differences inoxygen concentration in these regions of the wafer; the provision ofsuch a process in which the formation of the denuded zone does notdepend upon the out-diffusion of oxygen; the provision of such a waferin which the thickness of the resulting denuded zone is essentiallyindependent of the details of the IC manufacturing process sequence; andthe provision of such a wafer in which the formation of the denuded zoneand the formation of the oxygen precipitates in the wafer bulk is notinfluenced by the thermal history and the oxygen concentration of theCzochralski-grown, single crystal silicon ingot from which the siliconwafer is sliced.

Briefly, therefore, the present invention is directed to a Cz singlecrystal silicon wafer having a front surface, a back surface, a centralplane between the front and back surfaces, a front surface layer whichcomprises the region of the wafer between the front surface and adistance, D₁, measured from the front surface and toward the centralplane, and a bulk layer which comprises the region of the wafer betweenthe central plane and the distance, D₁, measured from the front surfaceof the wafer. In addition, the wafer has a non-uniform distribution ofcrystal lattice vacancies (one type of intrinsic point defect whichforms in single crystal silicon) with the concentration of vacancies inthe bulk layer being greater than the concentration of vacancies in thefront surface layer with the difference in concentration being such thatupon being subjected to the heat treatment cycles of essentially anyelectronic device manufacturing process, the wafer will form a denudedzone in the front surface layer and have a sufficient density of oxygenprecipitates for IG purposes in the bulk layer.

The present invention is also directed to a wafer having a frontsurface, a back surface, and a central plane between the front and backsurfaces, the wafer having a front surface layer consisting of theregion of the wafer within a distance, D₂, of the front surface whereinD₂ is no more than about 15 micrometers, a bulk layer comprising theregion of the wafer between the central plane the front surface layer,and a denuded zone which comprises the front surface layer and a portionof the bulk layer. The wafer is characterized in that it has asubstantially uniform oxygen concentration in the bulk layer, oxygenprecipitates in the portion of the bulk layer which is outside thedenuded zone, and a substantial absence of oxygen precipitates in thedenuded zone.

The present invention is further directed to a process for heat-treatinga Cz, single crystal silicon wafer to influence the precipitationbehavior of oxygen in the wafer in a subsequent thermal processing step.The wafer has a front surface, a back surface, a central plane betweenthe front and back surfaces, a vacancy sink at the front surface, afront surface layer which comprises the region of the wafer between thefront surface and a distance, D₁, measured from the front surface andtoward the central plane, and a bulk layer which comprises the region ofthe wafer between the central plane and the distance, D₁, measured fromthe front surface of the wafer. In the process, the wafer is subjectedto a heat-treatment to form crystal lattice vacancies throughout thewafer. The concentration of vacancies in the heat-treated wafer is thenreduced by controlling the cooling rate of the heat-treated wafer toproduce a wafer having a vacancy concentration profile in which the peakdensity is at or near the central plane with the concentration generallydecreasing in the direction of the front surface of the wafer. Thedifference in the concentration of vacancies in the front surface layerand the bulk layer is such that a thermal treatment of the wafer at atemperature in excess of 750° C. will lead to the formation of a denudedzone in the front surface layer and oxygen clusters or precipitates inthe bulk zone with the concentration of the oxygen clusters orprecipitates being primarily dependant upon the concentration ofvacancies in the bulk layer.

Other objects and features of this invention will be in part apparentand in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of the process of the present invention.

FIG. 2 is a photograph of a cross-section of a wafer (sample 4-7) whichwas processed as described in Example 1.

FIG. 3 is a photograph of a cross-section of a wafer (sample 4-8) whichwas subjected to the series of steps described in Example 1.

FIG. 4 is a photograph of a cross-section of a wafer (sample 3-14) whichwas subjected to the series of steps described in Example 1.

FIG. 5 is a graph of the log of platinum concentration (atoms/cm³)versus depth from the surface of a wafer (sample 4-7) which wassubjected to the series of steps set forth in Example 1.

FIG. 6 is a photograph of a cross-section of a wafer (sample 3-4) whichwas subjected to the series of steps set forth in Example 2.

FIG. 7 is a photograph of a cross-section of a wafer (sample 3-5) whichwas subjected to the series of steps set forth in Example 2.

FIG. 8 is a photograph of a cross-section of a wafer (sample 3-6) whichwas subjected to the series of steps set forth in Example 2.

FIG. 9 is a photograph of a cross-section of a wafer (sample 1-8) whichwas subjected to the series of steps set forth in Example 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, an ideal precipitating waferhas been discovered which, during essentially any electronic devicemanufacturing process, will form a denuded zone of sufficient depth anda wafer bulk containing a sufficient density of oxygen precipitates forIG purposes. Advantageously, this ideal precipitating wafer may beprepared in a matter of minutes using tools which are in common use inthe semiconductor silicon manufacturing industry. This process creates a“template” in the silicon which determines or “prints” the manner inwhich oxygen will precipitate during the electronic device manufacturingprocess.

The starting material for the ideal precipitating wafers of the presentinvention is a single crystal silicon wafer which has been sliced from asingle crystal ingot grown in accordance with conventional Czochralskicrystal growing methods. Such methods, as well as standard siliconslicing, lapping, etching, and polishing techniques are disclosed, forexample, in F. Shimura, Semiconductor Silicon Crystal Technology,Academic Press, 1989, and Silicon Chemical Etching, (J. Grabmaier ed.)Springer-Verlag, New York, 1982 (incorporated herein by reference).

Czochralski-grown silicon typically has an oxygen concentration withinthe range of about 5×10¹⁷ to about 9×10¹⁷ atoms/cm³ (ASTM standardF-121- 83). Because the oxygen precipitation behavior of the waferbecomes essentially decoupled from the oxygen concentration in the idealprecipitating wafer, the starting wafer may advantageously have anoxygen concentration falling anywhere within the range attainable by theCzochralski process.

Depending upon the cooling rate of the single crystal silicon ingot fromthe temperature of the melting point of silicon (about 1410° C.) throughthe range of about 750° C. to about 350° C., oxygen precipitatenucleation centers may form in the single crystal silicon ingot fromwhich the wafer is sliced. The presence or absence of these nucleationcenters in the starting material is not critical to the presentinvention provided, however, these centers are capable of beingdissolved by heat-treating the silicon at temperatures not in excess ofabout 1300° C. Certain heat-treatments, such as annealing the silicon ata temperature of about 800° C. for about four hours, can stabilize thesecenters such that they are incapable of being dissolved at temperaturesnot in excess of about 1150° C. The detection limit for oxygenprecipitates is currently about 5×10⁶ precipitates/cm³. The presence (ordensity) of oxygen precipitation nucleation centers cannot be directlymeasured using presently available techniques. As previously discussed,preexisting oxygen precipitate nucleation centers in the silicon can bestabilized and precipitates can be grown at these sites by subjectingthe silicon to an oxygen precipitation heat treatment. Thus, thepresence of these nucleation centers can indirectly be measured after anoxygen precipitation heat treatment, e.g., annealing the wafer at atemperature of 800° C. for four hours and then at a temperature of 1000°C. for sixteen hours in an oxygen containing atmosphere.

Substitutional carbon, when present as an impurity in single crystalsilicon has the ability to catalyze the formation of oxygen precipitatenucleation centers. For this and other reasons, therefore, it ispreferred that the single crystal silicon starting material have a lowconcentration of carbon. That is, the single crystal silicon should havea concentration of carbon which is less than about 5×10¹⁶ atoms/cm³,preferably which is less than 1×10¹⁶ atoms/cm³, and more preferably lessthan 5×10¹⁵ atoms/cm³.

Referring now to FIG. 1, the starting material for the idealprecipitating wafer of the present invention, single crystal siliconwafer 1, has a front surface 3, a back surface 5, and an imaginarycentral plane 7 between the front and back surfaces. The terms “front”and “back” in this context are used to distinguish the two major,generally planar surfaces of the wafer; the front surface of the waferas that term is used herein is not necessarily the surface onto which anelectronic device will subsequently be fabricated nor is the backsurface of the wafer as that term is used herein necessarily the majorsurface of the wafer which is opposite the surface onto which theelectronic device is fabricated. In addition, because silicon waferstypically have some total thickness variation, warp and bow, themidpoint between every point on the front surface and every point on theback surface may not precisely fall within a plane; as a practicalmatter, however, the TTV, warp and bow are typically so slight that to aclose approximation the midpoints can be said to fall within animaginary central plane which is approximately equidistant between thefront and back surfaces.

In a first embodiment of the process of the present invention thesurface of wafer 1 is modified in step S₁, to create a sink for crystallattice vacancies. Preferably, the surface is modified by forming asuperficial oxide layer 9 which envelopes wafer 1. Experimental evidenceobtained to-date suggests that oxide layers having a thickness of atleast about 30 Angstroms effectively serve as a sink for crystal latticevacancies for those applications considered to-date and that little orno additional benefit would be obtained for these applications byincreasing the thickness of the oxide layer, for example, to 50 Ångstroms or more. Experimental evidence obtained to-date furthersuggests that the oxide layer thickness should preferably be greaterthan the thickness of a native oxide layer (the silicon dioxide layerwhich naturally forms on silicon when silicon is exposed to ambient airand which typically has a thickness of about 15 Å ngstroms). In general,therefore, the sink for crystal lattice vacancies is formed by growingan oxide layer on the surface of the wafer to a thickness which isgreater than the thickness of a native oxide layer and, depending uponthe application, to a thickness of least about 20 Å ngstroms, at leastabout 25 Å ngstroms, or even at least about 30 Å ngstroms. Superficialoxide layers having these thicknesses may be obtained, for example, byannealing the wafer at an elevated temperature in an oxygen containingatmosphere.

In step S₂, the wafer is subjected to a heat-treatment step in which thewafers are heated to an elevated temperature to form and therebyincrease the number density of crystal lattice vacancies 13 in wafer 1.Preferably, this heat-treatment step is carried out in a rapid thermalannealer in which the wafers are rapidly heated to a target temperatureand annealed at that temperature for a relatively short period of time.In general, the wafer is subjected to a temperature in excess of 1150°C., preferably at least 1175° C., more preferably at least about 1200°C., and most preferably between about 1200° C. and 1275° C.

In the first embodiment of the present invention, the rapid thermalannealing step is carried out in the presence of a nitriding atmosphere,that is, an atmosphere containing nitrogen gas (N₂) or anitrogen-containing compound gas such as ammonia. An increase in vacancyconcentration throughout the wafer is achieved nearly, if notimmediately, upon achieving the annealing temperature; annealing thewafer at this temperature in the nitriding atmosphere will furtherincrease the vacancy concentration. Thus, the wafer will generally bemaintained at this temperature for at least one second, typically for atleast several seconds (e.g., at least 3) or even several tens of secondsof seconds and, depending upon the desired characteristics of the waferand the atmosphere in which the wafer is being annealed, for a periodwhich may range up to about 60 seconds (which is near the limit forcommercially available rapid thermal annealers).

In general, annealing the wafer in a rapid thermal annealer in thepresence of a nitriding atmosphere produces a non-uniform vacancyconcentration (number density) profile in the wafer with the peakconcentration occurring within about 50 to 100 micrometers of thesurface which is exposed to the nitriding gas and a lesser andrelatively uniform concentration in the wafer bulk. If the front andback surfaces of the wafer are exposed to the nitriding atmosphereduring the rapid thermal annealing step, therefore, the resulting waferwill have a vacancy concentration (number density) profile which isgenerally “U-shaped” for a cross-section of the wafer, that is, amaximum concentration will occur within several micrometers of each ofthe front and back surfaces and a relatively constant and lesserconcentration will occur throughout the wafer bulk.

Experimental evidence obtained to-date suggests that the atmosphere inwhich the rapid thermal annealing step is carried out should have anessential absence of oxygen in order for the build-up of vacancies tooccur. That is, the atmosphere should have a total absence of oxygen ora partial pressure of oxygen which is insufficient to inject sufficientquantities of silicon self-interstitial atoms which suppress thebuild-up of vacancy concentrations. While the lower limit of oxygenconcentration has not been precisely determined, it has beendemonstrated that for partial pressures of oxygen of 0.1 atmospheres, noincrease in vacancy concentration is observed. Thus, it is preferredthat the atmosphere have a partial pressure of oxygen of less than 0.1atm, more preferably less than 0.05 atm., and most preferably less than0.001 atm.

In addition to causing the formation of crystal lattice vacancies, therapid thermal annealing step causes the dissolution of any unstabilizedoxygen precipitate nucleation centers which are present in the siliconstarting material. These nucleation centers may be formed, for example,during the growth of the single crystal silicon ingot from which thewafer was sliced or as a consequence of some other event in the previousthermal history of the wafer or of the ingot from which the wafer issliced. Thus, the presence or absence of these nucleation centers in thestarting material is not critical, provided these centers are capable ofbeing dissolved during the rapid thermal annealing step.

The rapid thermal anneal may be carried out in any of a number ofcommercially available rapid thermal annealing (“RTA”) furnaces in whichwafers are individually heated by banks of high power lamps. RTAfurnaces are capable of rapidly heating a silicon wafer, e.g., they arecapable of heating a wafer from room temperature to 1200° C. in a fewseconds. One such commercially available RTA furnace is the model 610furnace available from AG Associates (Mountain View, CA).

Crystal lattice vacancies, like metals and other elements, are capableof diffusing through single crystal silicon with the rate of diffusionbeing temperature dependant. For example, crystal lattice vacancies arerelatively mobile at temperatures in the vicinity of the temperature atwhich the wafer is annealed in the rapid thermal annealing step whereasthey are essentially immobile for any commercially practical time periodat temperatures of as much as 700° C. Experimental evidence obtainedto-date suggests that the diffusion rate of vacancies slows considerablyat temperatures less than about 700° C. and perhaps as great as 800° C.,900° C., or even 1,000° C., the vacancies can be considered to beimmobile for any commercially practical time period.

Upon completion of step S₂, the wafer is rapidly cooled in step S₃through the range of temperatures at which crystal lattice vacancies arerelatively mobile in the single crystal silicon. As the temperature ofthe wafer is decreased through this range of temperatures, the vacanciesdiffuse to the oxide layer 9 and become annihilated, thus leading to achange in the vacancy concentration profile with the extent of changedepending upon the length of time the wafer is maintained at atemperature within this range. If the wafer were held at thistemperature within this range for an infinite period of time, thevacancy concentration would once again become substantially uniformthroughout wafer bulk 11 with the concentration being an equilibriumvalue which is substantially less than the concentration of crystallattice vacancies immediately upon completion of the heat treatmentstep. By rapidly cooling the wafer, however, a non-uniform distributionof crystal lattice vacancies can be achieved with the maximum vacancyconcentration being at or near central plane 7 and the vacancyconcentration decreasing in the direction of the front surface 3 andback surface 5 of the wafer. In general, the average cooling rate withinthis range of temperatures is at least about 5° C. per second,preferably at least about 20° C. per second, more preferably at leastabout 50° C. per second, still more preferably at least about 100° C.per second, with cooling rates in the range of about 100° C. to about200° C. per second being presently most preferred. Once the wafer iscooled to a temperature outside the range of temperatures at whichcrystal lattice vacancies are relatively mobile in the single crystalsilicon, the cooling rate does not appear to significantly influence theprecipitating characteristics of the wafer and thus, does not appear tobe narrowly critical.

Conveniently, the cooling step may be carried out in the same atmospherein which the heating step is carried out. Alternatively, the wafer maybe cooled in an oxygen containing atmosphere.

After the wafer has been cooled, it is ready for use in an electronicdevice manufacturing process. Typically, in one of the first steps ofsuch process wafers are loaded into a furnace which is at a temperatureof about 800° C. When loaded into a furnace at this temperature, thepreviously cooled wafer will have separate zones which behavedifferently with respect to oxygen precipitation. In the high vacancyregions (the wafer bulk), oxygen clusters rapidly as the wafer entersthe furnace. By the time the loading temperature is reached, theclustering process is finished and a distribution of clusters is reachedwhich depends only upon the initial concentration of vacancies. In thelow vacancy regions (near the wafer surfaces), the wafer behaves like anormal wafer which lacks pre-existing oxygen precipitate nucleationcenters; that is, oxygen clustering is not observed. As the temperatureis increased above 800° C. or if the temperature remains constant, theclusters in the vacancy rich zone grow into precipitates and are therebyconsumed whereas in the vacancy lean zone, nothing happens. By dividingthe wafer into various zones of vacancy concentration, a template iseffectively created through which is written an oxygen precipitatepattern which is fixed the moment the wafer is loaded into the furnace.

After oxygen precipitation heat-treatment step S₄ (e.g., annealing thewafer at a temperature of 800° C. for four hours and then at atemperature of 1000° C. for sixteen hours in an oxygen containingatmosphere), the resulting depth distribution of oxygen precipitates inthe wafer is characterized by clear regions of oxygen precipitate-freematerial (denuded zones) 15 and 15′ extending from the front surface 3and back surface 5 to a depth t, t′, respectively. Between these oxygenprecipitate-free regions, is a region 17 containing a substantiallyuniform density of oxygen precipitates.

The concentration of oxygen precipitates in region 17 is primarily afunction of the heating step and secondarily a function of the coolingrate. In general, the concentration of oxygen precipitates increaseswith increasing temperature and increasing annealing times in theheating step with precipitate densities in the range of about 1×10⁷ toabout 5×10¹⁰ precipitates/cm³ being routinely obtained.

The depth t, t′ from the front and back surfaces, respectively, ofoxygen precipitate-free material (denuded) zones 15 and 15′ is primarilya function of the cooling rate through the temperature range at whichcrystal lattice vacancies are relatively mobile in silicon. In general,the depth t, t′, decreases with decreasing cooling rates with denudedzone depths of at least about 20, 30, 40, 50, 70 or even 100 micrometersbeing attainable. Significantly, the depth of the denuded zone isessentially independent of the details of the electronic devicemanufacturing process and, in addition, does not depend upon theout-diffusion of oxygen as is conventionally practiced. While the heattreatments employed in this process may result in the out-diffusion of asmall amount of oxygen from the surface of the front and back surfacesof the wafer, the wafer bulk at depths of at least 5 micrometers fromthe wafer surface will have a substantially uniform oxygenconcentration. Nevertheless, a denuded zone of substantially greaterdepth can be formed as a consequence of the cooling step which causedthe formation of a non-uniform distribution of crystal lattice vacanciesin the wafer.

Unlike prior methods used to form denuded zones, it is unnecessary tosubject the single crystal silicon to a high temperature step to causethe outdiffusion of oxygen in regions near the surface of the silicon.Such high temperature steps when carried out for the sole purpose offorming a denuded zone add significant cost to the silicon wafer. Thus,the wafer of the present invention will have a denuded zone and asubstantially uniform oxygen concentration as a function of depth fromthe silicon surface. For example, the wafer will have a uniformconcentration of oxygen from the center of the wafer to regions of thewafer which are within about 15 microns of the silicon surface, morepreferably from the center of the silicon to regions of the wafer whichare within about 10 microns of the silicon surface, even more preferablyfrom the center of the silicon to regions of the wafer which are withinabout 5 microns of the silicon surface and most preferably from thecenter of the silicon to regions of the wafer which are within 3 micronsof the silicon surface. In this context, substantially uniform oxygenconcentration shall mean a variance in the oxygen concentration of nomore than about 50%, preferably no more than about 20% and mostpreferably no more than about 10%.

In a second embodiment of the present invention, a non-nitridingatmosphere is used instead of the nitriding atmosphere used in theheating (rapid thermal annealing) and cooling steps of the firstembodiment. Suitable non-nitriding atmospheres include argon, helium,neon, carbon dioxide, and other such non-oxidizing, non-nitridingelemental and compound gases.

When argon is used as the atmosphere in the heating (rapid thermalannealing) step, the increase in vacancy concentration throughout thewafer is achieved nearly, if not immediately, upon achieving theannealing temperature. Maintaining the wafer at an establishedtemperature for additional time does not appear, based upon experimentalevidence obtained to-date, to lead to an increase in vacancyconcentration. This behavior is different from what has been observedwhen a nitriding atmosphere is used; in nitriding atmospheres, vacancyconcentration appears to increase as a function of time at anestablished annealing temperature. Another difference observed whenargon is used is the profile of the resulting vacancy concentration(number density); when the atmosphere is argon the resulting profile isrelatively constant from the front to the back of the wafer whereas theresulting profile is “U-shaped” when a nitriding atmosphere is used.

Experimental evidence obtained to-date further suggests that oxide layer9 may not be required for all applications in which argon or othernon-nitriding atmospheres are used for the heating (rapid thermalannealing) step. That is, for these applications a silicon surfacehaving no more than a native oxide may serve as an effective sink forcrystal lattice vacancies. Depending upon the atmosphere employed duringthe rapid thermal annealing step and the desired oxygen precipitationprofile of the wafer, therefore, the modified surface layer be formedomitted or formed only upon the side of the wafer at which the denudedzone is desired, i.e., front surface 3 of the wafer (see FIG. 1).Optionally, therefore, back surface 5 of wafer 1 may be shielded duringthe formation of modified layer 9 to produce a wafer having a modifiedlayer at the surface of the front surface 3, but not the back surface 5of the wafer.

The starting material may be a polished silicon wafer, or alternatively,a silicon wafer which has been lapped and etched, but not polished. Ifan epitaxial layer is to be deposited upon the wafer, the process of thepresent invention may be carried out either before or after theepitaxial deposition. If carried out before, it may be desirable tostabilize the oxygen precipitate nucleation centers in the wafer afterthe process of the present invention and before the epitaxialdeposition. If carried out after, it may be desirable to carry out theprocess of the present invention in the epitaxial reactor immediatelyafter the epitaxial deposition, provided the cooling rates required bythe process of the present invention can be achieved.

The measurement of crystal lattice vacancies in single crystal siliconcan be carried out by platinum diffusion analysis. In general, platinumis deposited on the samples and diffused in a horizontal surface withthe diffusion time and temperature preferably being selected such that,the Frank-Turnbull mechanism dominates the platinum diffusion, but whichis sufficient to reach the steady-state of vacancy decoration byplatinum atoms. For wafers having vacancy concentrations which aretypical for the present invention, a diffusion time and temperature of730° C. for 20 minutes would be suitable. In addition, to minimize apossible influence by silicidation processes, the platinum depositionmethod preferably results in a surface concentration of less than onemonolayer. Platinum diffusion techniques are described elsewhere, forexample, by Zimmermann and Ryssel, “The Modeling of Platinum DiffusionIn Silicon Under Non-Equilibrium Conditions,” J. ElectrochemicalSociety, vol. 139, p. 256 (1992); Zimmermann, Goesele, Seilenthal andEichiner, “Vacancy Concentration Wafer Mapping In Silicon,” Journal ofCrystal Growth, vol. 129, p. 582 (1993); Zimmermann and Falster,“Investigation Of The Nucleation of Oxygen Precipitates in CzochralskiSilicon At An Early Stage,” Appl. Phys. Lett., vol. 60, p. 3250 (1992);and Zimmermann and Ryssel, Appl. Phys. A, vol. 55, p. 121 (1992).

The following examples will illustrate the invention.

EXAMPLE 1

Silicon single crystals were pulled by the Czochralski method, slicedand polished to form silicon wafers. These wafers were then subjected toa surface oxidation step (S₁), rapid thermal annealing step in nitrogenor argon (S₂), rapidly cooled (S₃), and subjected to an oxygenstabilization and growth step (S₄) under the conditions set forth inTable I. The initial oxygen concentration of the wafers (O_(i)) beforesteps S₁-S₄, the oxygen precipitate density in the bulk of the wafersafter step S₄ (OPD), and the depth of the denuded zone after step S₄(DZ) are also reported in Table I.

TABLE I Sample 4-7 4-8 3-14 S₁ 15 min at 15 min at none 1,000° C. 1,000°C. in O₂ in O₂ S₂ 35 seconds 35 seconds 35 seconds at 1250° C. at 1250°C. at 1250° C. in N₂ in Ar in Ar S₃ 100° C./sec 100° C./sec 100° C./secS₄ 4 hr at 4 hr at 4 hr at 800° C. + 800° C. + 800° C. + 16 hr at 16 hrat 16 hr at 1,000° C. 1,000° C. 1,000° C. in N₂ in N₂ in N₂ O_(i) 7 ×10¹⁷ 6.67 × 10¹⁷  7.2 × 10¹⁷ (atoms/cm³) OPD 1 × 10¹⁰  4.4 × 10⁹ 1.69 ×10¹⁰ (atoms/cm³) DZ 70 95 0 (depth in μm)

FIGS. 2, 3, and 4 are photographs of cross-sections of the resultingwafers at a magnification of 200×; with sample 4-7 being shown in FIG.2, sample 4-8 being shown in FIG. 3, and sample 3-14 being shown in FIG.4.

In addition, the concentration of crystal lattice vacancies in thesample 4-7 was mapped using a platinum diffusion technique. A plot ofplatinum concentration versus depth from the surface of the wafer (adepth of 0 micrometers corresponding to the front side of the wafer)appears in FIG. 5.

EXAMPLE 2

To demonstrate that the process of the present invention is relativelyindependent of oxygen concentration for Czochralski-grown siliconwafers, three wafers having different oxygen concentrations weresubjected to the same series of steps described in Example 1. Theconditions for each of these steps, the initial oxygen concentration ofthe wafers (O_(i)) before steps S₁-S₄, the oxygen precipitate density(OPD) in the bulk of the wafers after step S₄, and the depth of thedenuded zone (DZ) after step S₄ as measured from the surface of thewafer are reported in Table I. FIGS. 6, 7, and 8 are photographs ofcross-sections of the resulting wafers at a magnification of 200×; withsample 3-4 being shown in FIG. 6, sample 3-5 being shown in FIG. 7, andsample 3-6 being shown in FIG. 8.

TABLE II Sample 3-4 3-5 3-6 S₁ 15 min at 15 min at 15 min at 1,000° C.1,000° C. 1,000° C. in O₂ in O₂ in O₂ S₂ 35 seconds 35 seconds 35seconds at 1250° C. at 1250° C. at 1250° C. in N₂ in N₂ in N₂ S₃ 125°C./sec 125° C./sec 125° C./sec S₄ 4 hr at 4 hr at 4 hr at 800° C. + 800°C. + 800° C. + 16 hr at 16 hr at 16 hr at 1,000° C. 1,000° C. 1,000° C.in N₂ in N₂ in N₂ O_(i) 6 × 10¹⁷ 7 × 10¹⁷ 8 × 10¹⁷ (atoms/cm³) OPD 4 ×10¹⁰ 1 × 10¹⁰ 6 × 10¹¹ (atoms/cm³) DZ ˜40 ˜40 ˜40 (depth in μm)

EXAMPLE 3

To demonstrate that the process of the present invention was relativelyindependent of the conditions used for the oxygen precipitatestabilization and growth step (S₄), a wafer (sample 1-8) having the sameinitial oxygen concentration was subjected to the same series of stepsdescribed in Example 2 for sample 3-4 except that a proprietary,commercial 16 M DRAM process was used as the oxygen precipitatestabilization and growth step (S₄). FIG. 9 is a photograph of thecross-section of the resulting wafer at a magnification of 200×. Afterstep S₄, samples 1-8 and 3-4 had comparable bulk oxygen precipitatedensities (7×10¹⁰/cm³ for sample 1-8 versus (4×10¹⁰/cm³ for sample 3-4)and comparable denuded zone depths (approximately 40 micrometers).

In view of the above, it will be seen that the several objects of theinvention are achieved.

As various changes could be made in the above compositions and processeswithout departing from the scope of the invention, it is intended thatall matter contained in the above description be interpreted asillustrative and not in a limiting sense.

We claim:
 1. A process for the preparation of a silicon wafer comprisinga front surface having an epitaxial layer deposited thereon, a backsurface, a central plane between the front and back surfaces, a frontsurface layer which comprises the region of the wafer between the frontsurface and a distance, D, measured from the front surface and towardthe central plane, and a bulk layer which comprises the region of thewafer between the central plane and front surface layer, the bulk regionfurther comprising a concentration of oxygen precipitates, wherein theprocess comprises: subjecting the wafer to a heat-treatment to formcrystal lattice vacancies in the front surface and bulk layers;controlling the cooling rate of the heat-treated wafer to produce awafer having a vacancy concentration profile in which the peak densityis at or near the central plane with the concentration generallydecreasing in the direction of the front surface of the wafer and thedifference in the concentration of vacancies in the front surface andbulk layers being such that a thermal treatment at a temperature inexcess of 750° C., is capable of forming in the wafer a denuded zone inthe front surface layer and oxygen clusters or precipitates in the bulkzone with the concentration of the oxygen clusters or precipitates inthe bulk layer being primarily dependant upon the concentration ofvacancies; subjecting the silicon wafer, to an oxygen precipitation heattreatment to cause the nucleation and growth of oxygen precipitates inthe bulk layer, to a size sufficient to stabilize the oxygenprecipitates such that they are incapable of being dissolved attemperatures not in excess of 1150° C., with the oxygen precipitateconcentration profile being formed according to the vacancyconcentration profile such that a denuded zone is formed in the frontsurface layer; and, depositing an epitaxial layer on at least onesurface of the oxygen precipitate stabilized wafer.
 2. The process ofclaim 1 wherein said heat-treatment to form crystal lattice vacanciescomprises heating the wafers to a temperature in excess of about 1175°C. in a non-oxidizing atmosphere.
 3. The process of claim 1 wherein saidheat-treatment to form crystal lattice vacancies comprises heating thewafers to a temperature in excess of about 1200° C. in a non-oxidizingatmosphere.
 4. The process of claim 1 wherein said heat-treatment toform crystal lattice vacancies comprises heating the wafers to atemperature in the range of about 1200° C. to about 1275° C. in anon-oxidizing atmosphere.
 5. The process of claim 1 wherein said coolingrate is at least about 20° C. per second through the temperature rangeat which crystal lattice vacancies are relatively mobile in silicon. 6.The process of claim 1 wherein said cooling rate is at least about 50°C. per second through the temperature range at which crystal latticevacancies are relatively mobile in silicon.
 7. The process of claim 1wherein said cooling rate is at least about 100° C. per second throughthe temperature range at which crystal lattice vacancies are relativelymobile in silicon.
 8. The process of claim 1 wherein the oxygenprecipitation heat treatment comprises heating the wafer to atemperature of at least about 800° C. for a period of time sufficient tocause the nucleation and growth of oxygen precipitates in the bulklayer, to a size sufficient to stabilize the oxygen precipitates suchthat they are incapable of being dissolved at temperatures not in excessof 1150° C.
 9. The process of claim 8 wherein the wafer is heated for aperiod of time of at least about 4 hours.
 10. The process of claim 8further comprising heating the wafer to a temperature of at least about1000° C. for at least about 16 hours.
 11. A process for the preparationof a silicon wafer comprising a front surface having an epitaxial layerdeposited thereon, a back surface, a central plane between the front andback surfaces, a front surface layer which comprises the region of thewafer between the front surface and a distance, D, measured from thefront surface and toward the central plane, and a bulk layer whichcomprises the region of the wafer between the central plane and frontsurface layer, the bulk region further comprising a concentration ofoxygen precipitates, wherein the process comprises: (a) subjecting thesilicon wafer to a first heat-treatment at a temperature of at leastabout 700° C. in an oxygen containing atmosphere to form a superficialsilicon dioxide layer which is capable of serving as a sink for crystallattice vacancies; (b) subjecting the product of step (a) to a secondheat-treatment at a temperature of at least about 1150° C. in anatmosphere having an essential absence of oxygen to form crystal latticevacancies in the bulk of the silicon; (c) cooling the product of step(b) from the temperature of said second heat treatment to a temperature,T₁, of about 800° C. at a rate which allows some, but not all, of thecrystal lattice vacancies to diffuse to the sink to produce a waferhaving a vacancy concentration profile in which the peak density is ator near the central plane with the concentration generally decreasing inthe direction of the front surface of the wafer; (d) Subjecting theproduct of step (c), to an oxygen precipitation heat treatment to causethe nucleation and growth of oxygen precipitates in the bulk layer, to asize sufficient to stabilize the oxygen precipitates such that they areincapable of being dissolved at temperatures not in excess of 1150° C.,with the oxygen precipitate concentration profile being formed accordingto the vacancy concentration profile such that a denuded zone is formedin the front surface layer; and, (e) depositing an epitaxial layer on atleast one surface of the product of step (d).
 12. The process of claim11 wherein the cooling rate is at least about 20° C. per second.
 13. Theprocess of claim 11 wherein the cooling rate is at least about 50° C.per second.
 14. The process of claim 11 wherein the cooling rate is atleast about 100° C. per second.
 15. The process of claim 11 wherein T₁is about 900° C.
 16. The process of claim 15 wherein the cooling rate isat least about 20° C. per second.
 17. The process of claim 15 whereinthe cooling rate is at least about 50° C. per second.
 18. The process ofclaim 15 wherein the cooling rate is at least about 100° C. per second.19. The process of claim 11 wherein T₁ is about 1,000° C.
 20. Theprocess of claim 19 wherein the cooling rate is at least about 20° C.per second.
 21. The process of claim 19 wherein the cooling rate is atleast about 50° C. per second.
 22. The process of claim 19 wherein thecooling rate is at least about 100° C. per second.
 23. The process ofclaims 11 wherein the oxygen precipitation heat treatment comprisesheating the wafer to a temperature of at least about 800° C. for aperiod of time sufficient to cause the nucleation and growth of oxygenprecipitates in the bulk layer, to a size sufficient to stabilize theoxygen precipitates such that they are incapable of being dissolved attemperatures not in excess of 1150° C.
 24. The process of claim 23wherein the wafer is heated for a period of time of at least about 4hours.
 25. The process of claims 23 further comprising heating the waferto a temperature of at least about 1000° C. for at least about 16 hours.